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Photonics Glossary and List of Formula Symbols

On this page, you first find the glossary of photonics terms, which is based on the definitions in the encyclopedia articles. These have been carefully worked out, such that they can be considered as authoritative.

Further below there is a list of common formular symbols.

Glossary of Photonics Terms

This page summarizes the definitions of photonics terms, as they appear below the headings of many of the encyclopedia pages.

A

Abbe number
a measure for the chromatic dispersion of a transparent material
ABCD matrix
a 2-by-2 matrix describing the effect of an optical element on a laser beam
absorbance
the logarithm with base 10 of the inverse transmittance
absorptance
the fraction of incoming radiant flux which is absorbed by an object
absorption
a process where light energy is converted to another form of energy
absorption coefficient
a logarithmic measure for the distributed absorption in a medium
absorption length
the length of which absorption reduces the optical intensity to 1/e of its initial value
acceptance angle in fiber optics
the maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide
achromatic optics
optical devices or setups with minimized chromatic aberrations
acousto-optic deflectors
acousto-optic devices for deflecting and scanning laser beams
acousto-optic frequency shifters
acousto-optic devices for shifting the optical frequency of a laser beam
acousto-optic modulators
optical modulators based on the acousto-optic effect
acousto-optic Q switches
acousto-optic modulators used for Q switching of lasers
acousto-optic tunable filters
acousto-optic devices which can be used for filtering light, controlled with an RF input
active mode locking
a technique of mode locking, based on active modulation of the intracavity losses or the round-trip phase change
additive-pulse mode locking
a technique for mode locking a laser, using a nonlinear interaction in an external resonator
adiabatic soliton compression
a pulse compression technique based on the adaptation of solitons to slowly varying propagation parameters
afocal optical systems
optical systems which output parallel rays for parallel input rays
alexandrite lasers
solid-state lasers based on alexandrite (Cr3+:BeAl2O4) crystals
alignment lasers
lasers used for alignment purposes
alignment sensitivity of optical resonators
the sensitivity of a laser, an optical resonator or some other device with respect to misalignment
all-solid-state lasers
laser systems containing solid-state devices only, in particular no discharge lamps or gas or dye lasers
amplification factor
the factor by which the power of a signal is amplified
amplified spontaneous emission
a process where spontaneously emitted radiation (luminescence) is amplified
amplifier chains
amplifiers consisting of several stages
amplifier noise
noise introduced to a signal in an amplifier device
amplitude-squeezed light
light with an intensity noise below the shot noise level
anamorphic prism pairs
prisms pairs for reshaping laser beams
anti-reflection coatings
optical thin-film coatings for reducing reflections from surfaces
aperture stops
apertures which limit the angular range of received input light of imaging systems
argon ion lasers
gas lasers based on light amplification in ionized argon in a gas discharge
arrayed waveguide gratings
optical filter or multiplexer devices based on arrays of waveguides
aspheric optics
optical elements with non-spherical surfaces
astigmatism
a kind of optical aberrations; alternatively, a defect of optical lenses or a property of laser beams
autocorrelators
devices for measuring the intensity or field autocorrelation function of light, mostly used for determining the duration of ultrashort pulses
avalanche photodiodes
photodiodes with internal signal amplification through an avalanche process

B

B integral
a measure of the nonlinear phase shift of light, e.g. in an amplifier
Babinet–Soleil compensators
adjustable waveplates (retarder plates) containing three birefringent plates
balanced photodetection
a method of photodetection which is sensitive to differences in optical powers but not to common noise
ball lenses
lenses which have the geometric form of a sphere
band gap
a range of energies for which there are no states of carriers in a medium
bandwidth
the width of some frequency or wavelength range
bandwidth–distance product
product of length and maximum signal bandwidth of a fiber-optic link
bandwidth-limited pulses
pulses with a duration as short as possible with their optical spectrum
beam collimators
devices for collimating optical beams
beam combining
a class of techniques for power scaling of laser sources by combining the outputs of multiple devices
beam divergence
a measure for how fast a laser beam expands far from its focus
beam dumps
devices for blocking and absorbing light beams
beam expanders
optical devices for modifying the beam radius of a collimated beam
beam homogenizers
devices for producing beam profiles with homogeneous intensity distribution
beam parameter product
product of the beam radius in a focus and the far-field beam divergence
beam pointing fluctuations
fluctuations of the propagation direction of a laser beam
beam profilers
devices for measuring the intensity profile of a laser beam
beam quality
a measure for how well a laser beam can be focused
beam radius
a measure of the transverse extension of a light beam
beam shapers
optical devices for modifying the shapes of laser beams
beam shutters
devices for blocking or releasing light beams
beam splitters
devices for splitting a laser beam into two or more beams
beam waist
location with minimum beam radius
beat note
an oscillation of the optical intensity arising from the superposition of light with different optical frequencies
Beer–Lambert law
a relation for the dependence of absorption coefficients on concentrations
bend losses
propagation losses in an optical fiber (or other waveguide) caused by bending
Berek compensators
tunable phase retardation plates containing a single uniaxial crystal
birefringence
the polarization dependence of the refractive index of a medium
birefringent phase matching
a technique of phase matching based on the birefringence of a crystal material
birefringent tuners
devices for wavelength tuning of lasers, based on birefringence and polarization rotation
bit error rate
average fraction of wrongly transmitted bits in a communication link
black coatings
coatings with strong light absorption
blue lasers
lasers emitting blue light
Bragg gratings
reflecting structures with a periodic refractive index modulation
Bragg mirrors
mirror structures based on Bragg reflection at a period structure
Brewster plates
coplanar plates inserted into beams at Brewster's angle
Brewster windows
transparent plates which are oriented at Brewster's angle such that parasitic reflection losses are minimized.
Brewster's angle
an angle of incidence at which there is no reflection of p-polarized light at an uncoated optical surface
brightness
a term mostly used in a qualitative way, related to the output power and beam quality of a laser; quantitatively: often used instead of radiance or luminance
brightness converters
optically pumped devices where the generated optical output has a higher brightness than the optical pump source
Brillouin scattering
a nonlinear scattering effect involving acoustic phonons
broad area laser diodes
laser diodes with a strongly asymmetric shape of the emitting region
bulk lasers
lasers based on bulk crystals as gain media

C

cameras
optical instruments for recording still or moving images
cardinal points
a set of points which characterize the function of an optical element or a group of elements in Gaussian optics
carrier–envelope offset
the offset between the optical phase and the maximum of the wave envelope of an optical pulse
causality
the concept of relating effects to causes, or the compatibility of phenomena with this concept
cavities
resonators for light (or for microwaves)
cavity dumping
a method for extracting intense short or ultrashort pulses from a laser, using an optical switch in the laser resonator
ceramic gain media
laser gain media which have a ceramic (polycrystalline) microscopic structure
chemiluminescence
luminescence arising from chemical reactions
chirp
time dependence of the instantaneous frequency of an optical pulse
chirped mirrors
Bragg-type dispersive mirrors with a spatial variation of the Bragg wavelength
chirped-pulse amplification
a technique for amplifying pulses to very high optical intensities while avoiding excessive nonlinear pulse distortions or optical damage
chromatic aberrations
image distortions caused by wavelength-dependent optical effects
chromatic dispersion
the frequency dependence of the phase velocity in a transparent medium
chromium-doped gain media
laser gain media doped with chromium ions
cladding mode strippers
devices which can remove light from a fiber cladding
cladding modes
modes in optical fibers (or other waveguides) which are not restricted to the region around the core
cleaning of optics
cleaning of optical elements
cleaving of fibers
preparing fiber ends with clean optical surfaces by controlled breaking
clustering
the tendency e.g. of laser-active ions in laser gain media to form clusters in their host medium
CO2 lasers
infrared lasers based on a gas mixture in which light is amplified by carbon dioxide molecules
coherence
a fixed phase relationship between the electric field values at different locations or at different times
coherence length
a measure of temporal coherence, expressed as the propagation distance over which the coherence significantly decays
coherence time
a measure of temporal coherence, expressed as the time over which the field correlation decays
coherent beam combining
a class of methods for beam combining, requiring mutual coherence of the combined beams
coherent states
a special kind of pure quantum-mechanical states of light
cold mirrors
mirrors which can reduce the heat load in an optical system by reflecting only visible radiation
collimated beams
laser beams with weak divergence
common-path interferometers
interferometers where the sensitivity to mechanical noise is reduced by largely using a common optical path for the interfering light beams
composite laser crystals
laser crystals consisting of several parts of different materials or with different chemical compositions (e.g. doping concentrations)
condensers
lenses or lens systems for conditioning light from an illumination source
confocal scanning microscopes
optical microscopes with enhanced depth resolution based on the confocal measurement principle
conjugate planes
pairs of planes where an optical system images one into the other and vice versa
continuous-wave operation
operation mode of a laser with continuous light emission
cooperative lasing
the phenomenon that laser action at one wavelength can facilitate lasing at another wavelength
core-less end caps
end pieces attached to fibers, containing no fiber core
corner cube prisms
prisms with three reflecting surfaces which act as retroreflectors
critical phase matching
phase matching of a nonlinear interaction by adjustment of a propagation direction
cross-phase modulation
a nonlinear effect where the optical intensity of one beam influences the phase change of another beam
cryogenic lasers
lasers where the gain medium is operated at cryogenic temperatures
crystal ovens
temperature-controlled ovens for keeping optical crystals at the desired operation temperature
crystalline mirrors
multilayer mirrors where the reflecting parts consist of a monocrystalline material
custom optics
optical elements made according to customer-specific specifications
cut-off wavelength
a wavelength above which a guided mode of a waveguide ceases to exist
cylindrical lenses
lenses which focus or defocus light only in one direction

D

dark current
a current from a photodetector which occurs even in the absence of a light input
decibel
a logarithmic measure for power ratios, applied e.g. to optical powers or to noise powers
deformable mirrors
mirrors which can be deformed in a controlled manner, e.g. for wavefront correction in adaptive optics
depolarization loss
losses of optical power in a laser resonator, caused by depolarization e.g. in a laser crystal
depth of field
the distance between the nearest and furthest objects that can be imaged with reasonably sharp focus for a given focus setting
depth of focus
the half width of the range of longitudinal positions in which a reasonable focus for a film or image sensor is achieved
detectivity
the inverse of the noise equivalent power
diameter–divergence product
product of the diameter (at a beam waist) and the divergence angle of a laser beam
diaphragms
circular (or approximately circular) openings through which light can travel
dichroic mirrors
mirrors with significantly different reflection or transmission properties at two different wavelengths
dichroism
wavelength-dependent transmission, alternatively polarization-dependent absorption
dielectric coatings
thin-film coatings made of transparent dielectric materials, e.g. for laser mirrors or anti-reflection coatings
dielectric mirrors
mirrors consisting of multiple thin layers of different transparent optical materials
differential mode delay
the range of time delay values for signals in a telecom fiber
diffraction
wave phenomena which occur when light waves hit some structure with variable transmission or phase changes
diffraction gratings
optical components containing a periodic structure which diffracts light
diffraction-limited beams
beams with a minimum possible beam divergence for a given waist radius
diffractive optics
optical elements with operation principles based on diffraction of light
diode bars
a type of semiconductor laser containing a one-dimensional array of broad-area emitters
diode lasers
semiconductor laser devices based on laser diodes
diode-pumped lasers
solid-state lasers which are pumped with laser diodes
diode stacks
arrangements of multiple diode bars, delivering very high output power
dioptric power
the inverse of the focal length
direct diode lasers
diode lasers which are directly used e.g. for material processing
dispersion
the dependence of the phase velocity in a medium on the optical frequency or the propagation mode
dispersion compensation
the control of the overall chromatic dispersion of a system by adding optical elements with a suitable amount of dispersion
dispersion compensation modules
modules used for dispersion compensation
dispersion-decreasing fibers
optical fibers where the chromatic dispersion changes monotonously along the propagation direction
dispersion management
the use of tailored chromatic dispersion to enhance the function of optical devices or systems
dispersion-shifted fibers
fibers with a non-standard zero dispersion wavelength
dispersive mirrors
mirrors which provide some amount of chromatic dispersion for the reflected beam
dispersive wave
a linearly propagating wave which is split off by a soliton wave under certain conditions
distance measurements with lasers
measurements of distances using laser light
distributed amplifiers
fiber amplifiers in fiber-optic data links, where the amplification occurs within a large length of transmission fiber
distributed Bragg reflector lasers
lasers containing distributed Bragg reflectors as end mirrors
distributed feedback lasers
lasers where the whole laser resonator consists of a periodic structure, in which Bragg reflection occurs
divided-pulse amplification
a method of amplifying intense ultrashort pulse while avoiding excessive nonlinear effects
doped insulator lasers
lasers with a solid-state gain medium containing a laser-active dopant
doping concentration
the concentration of some dopant, e.g. of laser-active ions in a laser gain medium
Doppler broadening
broadening of the linewidth of atomic transitions, caused by random motion of the emitting atoms
Doppler cooling
a technique of laser cooling based on velocity-dependent absorption processes
Doppler limit
a limit for the temperature which is achievable with certain laser cooling techniques
double-clad fibers
optical fibers with different waveguide structures for pump and signal light
double pulses
two closely spaced optical pulses
dye lasers
lasers based on liquid or solid dyes as gain media

E

edge-emitting semiconductor lasers
semiconductor lasers emitting light along the plane of the wafer
effective mode area
a quantitative measure of the area which a waveguide or fiber mode effectively covers in the transverse dimensions
effective nonlinear coefficient
a coefficient for quantifying the strength of a nonlinear interaction
effective refractive index
a number quantifying the phase delay per unit length in a waveguide, relative to the phase delay in vacuum
effective transition cross sections
a modified type of transition cross sections which apply to optical transitions between Stark level manifolds
electro-optic effect
the phenomenon that the refractive index of a material can be modified with an electric field
electro-optic modulators
optical modulators based on the electro-optic effect
electro-optic sampling
an optical sampling technique based on the electro-optic effect
electro-optics
the science and technology around the electro-optic effect; alternatively: optoelectronics or even the whole area of photonics
electroabsorption modulators
optical modulators based on the Franz–Keldysh effect
electroluminescence
luminescence arising from electrical influences
electronic dispersion compensation
a method for mitigating the effects of chromatic dispersion in fiber-optic communication links with electronic components in the receiver
emissivity
a measure for how strongly an object emits and absorbs thermal radiation
end pumping
a technique of optically pumping a laser medium in a direction along the laser beam
energy transfer
the phenomenon that dopant ions in laser-active media can exchange excitation energy among each other
enhancement cavities
optical cavities for exploiting the resonant enhancement of the power of circulating light
entrance and exit pupil
the apparent apertures of an optical system, seen from the object or image side
erbium-doped fiber amplifiers
fiber amplifiers based on erbium-doped optical fibers
erbium-doped gain media
laser gain media doped with erbium ions
etalons
monolithic interferometric devices containing two parallel reflecting surfaces
excimer lasers
lasers where optical amplification occurs in a plasma containing excited dimers (or other molecules) with an anti-binding electronic ground state
excited-state absorption
absorption of light by ions or atoms in an excited electronic state, rather than in the electronic ground state
external-cavity diode lasers
non-monolithic diode lasers where the laser cavity (resonator) is completed with external optical elements
eye protection
the protection of eyes against laser hazards
eye-safe lasers
lasers emitting in a wavelength region with relatively low hazards for the human eye

F

f-number
a measure for the open aperture of a photographic objective
Fabry–Pérot interferometers
interferometers consisting of two highly reflecting mirrors, forming a standing-wave resonator
Faraday isolators
optical isolators based on the Faraday effect
Faraday rotators
devices which can rotate the polarization state of light, exploiting the Faraday effect
femtosecond lasers
lasers emitting light pulses with durations between a few femtoseconds and hundreds of femtoseconds
few-mode fibers
fibers supporting only few guided modes
fiber amplifiers
optical amplifiers with doped fibers as gain media
fiber Bragg gratings
reflective structures in the core of an optical fiber with a periodic or aperiodic perturbation of the effective refractive index
fiber bundles
bundles of optical fibers
fiber cables
cables containing one or several optical fibers
fiber cladding
the area(s) around the core of an optical fiber
fiber cleavers
instruments for preparing fiber ends with clean optical surfaces
fiber collimators
devices for collimating the light coming from a fiber, or for launching collimated light into the fiber
fiber connectors
connectors used as terminations of optical fiber cables
fiber core
the region in an optical fiber which guides light
fiber-coupled diode lasers
diode laser devices where the generated light is coupled into an optical fiber
fiber couplers
fiber devices for coupling light from one or several input fibers to one or several output fibers, or from free space into a fiber
fiber fabrication
the fabrication of optical fibers
fiber fuse
the effect that an optical fiber "burns" from the output end under the influence of an intense laser beam launched into the other end
fiber joints
permanent or removable connections between fiber ends
fiber lasers
lasers with a doped fiber as gain medium, or (sometimes) just lasers where most of the laser resonator is made of fibers
fiber launch systems
optical systems for launching light into optical fibers
fiber loop mirrors
fiber loops which act as mirrors
fiber-optic links
optical communication links where the signal light is transported in fibers
fiber-optic networks
communication systems consisting of fiber-optic links
fiber-optic plates
transparent plates containing many optical fibers
fiber-optic pump combiners
fiber-optic couplers for sending pump and signal light into an amplifier or laser fiber
fiber-optic sensors
optical sensors based on fiber devices
fiber-optic tapers
fiber-optic plates which are tapered for obtaining some image magnification
fiber optics
optics based on optical fibers
fiber patch cables
a protected and connectorized fiber-optic cable
fiber polarization controllers
devices which allow one to control the state of polarization of light within fibers
fiber simulation software
software for numerical simulations on fiber devices
fiber to the home
the technology of providing data connections to private homes via optical fibers rather than electric cables
fibers
a kind of long and thin optical waveguides which can be bent to some degree
field lenses
lenses in imaging instruments which affect the field of view
field of view
the angular extent of the field which can be observed with an optical instruments or the eye
field stops
apertures which limit the field of view of imaging systems
finesse
the free spectral range divided by the FWHM width of the resonances of an optical resonator
first surface mirrors
metal-coated mirrors reality metallic mineral coating is on the front surface of the mirror substrate
Fizeau interferometers
interferometers for investigating optical surfaces
flat-top beams
a light beam with a flat intensity profile
fluence
optical energy per unit area
fluorescence
luminescence arising from irradiation with light
fluorescence microscopy
a technique for acquiring microscopic images using fluorescence which is excited in the sample
fluorescence spectroscopy
spectroscopy which is based on the analysis of fluorescence light
fluorescent lamps
lamps which emit fluorescent light, usually generated by irradiation of a phosphor with light from an electric gas discharge
fluoride fibers
optical fibers based on fluoride glasses
fluoride glasses
glasses made from compounds of fluorine e.g. with zirconium, aluminum or indium
focal distance
the distance between an objective and its focal point
focal length
a measure of how strongly an optical system focuses or defocuses light
focal plane arrays
arrays of light detectors placed in the focal plane of an imaging system
focal points and focal planes
points to which parallel input rays are concentrated by an optical system, and the planes going through those points
forbidden transitions
transitions between different energy levels of some atoms or ions for which dipole transitions are suppressed via symmetries
four-level and three-level gain media
laser gain media without/with reabsorption from the lower laser level
four-wave mixing
an interaction of light waves based on a χ(3) nonlinearity
Fourier optics
the description of light propagation based on transverse spatial Fourier transforms
Fourier transform spectroscopy
a method for spectroscopy rare an optical spectrum is computed from raw data with a Fourier transform
free electron lasers
laser devices where light amplification occurs by interaction with fast electrons in an undulator
free-space optical communications
optical data transmission through free space, usually through air or vacuum
free spectral range
frequency spacing of the axial modes of an optical resonator
frequency combs
optical spectra consisting of equidistant lines
frequency doubling
the phenomenon that an input wave in a nonlinear material can generate a wave with twice the optical frequency
frequency metrology
the field of technology dealing with precise frequency measurements
frequency noise
noise of the instantaneous frequency of an oscillating signal
frequency quadrupling
the phenomenon that an input laser beam generates a beam with four times the optical frequency
frequency-resolved optical gating
a technique for the complete characterization of ultrashort optical pulses
frequency tripling
the phenomenon that an input laser beam generates a beam with three times the optical frequency
Fresnel equations
equations for the amplitude coefficients of transmission and reflection at the interface between two transparent homogeneous media
Fresnel lenses
lenses where the surface profile has annular steps
Fresnel number
a parameter determining the regime of diffraction effects
Fresnel reflections
light reflections at optical interfaces
F├╝chtbauer–Ladenburg equation
an equation used for calculating emission cross sections of laser gain media
fundamental mode locking
mode locking with a single pulse circulating in the laser resonator
fusion splicing of fibers
a technique of making low-loss fiber joints by fusing fiber ends together

G

gain
a measure of the strength of optical amplification
gain bandwidth
the width of the optical frequency range in which significant gain is available from an amplifier
gain clamping
a firm limitation of the gain of some amplifier or laser, usually caused by lasing action
gain efficiency
small-signal gain of an optical amplifier per unit pump power or per unit stored energy
gain equalization
a technique for making the gain spectrum of an optical amplifier device flatter over a certain optical frequency range
gain guiding
guiding of light based on spatially inhomogeneous optical gain
gain media
media for laser amplification
gain narrowing
the phenomenon that the bandwidth of light can be reduced during amplification in a medium with a limited gain bandwidth
gain saturation
the phenomenon that the gain of an amplifier is reduced for high input signal powers
gain switching
a technique for generating short optical pulses in a laser by modulating the laser gain
gas lasers
lasers with a gas (or plasma) as gain medium
Gaussian beams
light beams where the electric field profile in a plane perpendicular to the beam axis can be described with a Gaussian function, possibly with an added parabolic phase profile
Gaussian optics
a framework for describing optical phenomena, which is based on geometrical optics and the paraxial approximation
Gaussian pulses
pulses with a temporal intensity profile which has a Gaussian shape
geometrical optics
a concept of optics which is based on the propagation of light rays, ignoring wave properties of light
Giles parameters
spectroscopic data concerning absorption and amplification in an active fiber
Gires–Tournois interferometers
linear optical resonators used for introducing chromatic dispersion
Gordon–Haus jitter
a timing jitter originating from fluctuations of the center frequency
Gouy phase shift
an additional phase shift occurring in the propagation of focused Gaussian beams
graded-index fibers
optical fibers with a continuously varying refractive index in the radial dimension
gradient-index lenses
lenses which utilize a radial variation of refractive index
green lasers
lasers emitting in the green spectral region
group delay
a measure of the time delay experienced by narrow-band light pulses in an optical device
group delay dispersion
the frequency dependency of the group delay, or (quantitatively) the corresponding derivative with respect to angular frequency
group index
the ratio of the vacuum velocity of light to the group velocity in a medium
group velocity
the velocity with which the envelope of a weak narrow-band optical pulse propagates in a medium
group velocity dispersion
the frequency dependence of the group velocity in a medium, or (quantitatively) the derivative of the inverse group velocity with respect to angular frequency
group velocity mismatch
the phenomenon that pulses become temporally separated in a medium due to different group velocities, or (quantitatively) the difference of inverse group velocities
guided waves
waves for which diffraction is suppressed by a waveguide structure

H

halogen lamps
incandescent lamps with a halogen filling, allowing for higher operation temperatures
harmonic mode locking
mode locking of a laser where multiple pulses are circulating in the laser resonator with equal temporal spacing
Haus Master equation
an analytical equation describing the evolution of ultrashort pulses in a laser resonator
helium–neon lasers
gas lasers based on a helium–neon mixture
Hermite–Gaussian modes
propagation modes or resonator modes which are described with Hermite–Gaussian functions
high brightness laser diodes
laser diodes with particularly high radiance (brightness)
high harmonic generation
the phenomenon that very high harmonics of an intense input laser beam are generated in a gas
high-power fiber lasers and amplifiers
fiber lasers and fiber amplifiers with high output powers of e.g. > 10  W
high-power lasers
lasers emitting very high optical powers
higher-order modes
transverse modes of a resonator or waveguide, excluding the fundamental (axial) modes
higher-order solitons
optical pulses in a nonlinear and dispersive medium which exhibit periodic oscillations of their temporal and spectral shape
hollow-core fibers
optical fibers with a hole on the fiber axis
holography
methods for recording and reconstructing three-dimensional images
homogeneous broadening
the increase in the linewidth of an atomic transition caused by effects which affect different radiating or absorbing atoms in the same way
homogeneous saturation
saturation of an absorption or gain, where the shape of the absorption or gain spectrum is not modified
hot mirrors
mirrors which can reduce the heat load in an optical system by reflecting infrared radiation
hyper Raman scattering
Raman scattering processes involving two pump photons
hyperfocal distance
the smallest focusing distance of an imaging instruments for which objects at infinity appear with reasonably sharp focus
hyperspectral imaging
imaging which provides images with an optical spectrum corresponding to each pixel

I

illuminance
the incident luminous flux per unit area on a surface
image intensifiers and image converters
instruments which can intensify images acquired under faint light conditions, or convert images to other wavelengths
image planes
planes which are conjugate to an object plane, so that objects are imaged there
image sensors
optoelectronic sensors which can be used for imaging
imaging
mapping objects points to image points; applications involving such methods
imaging with a lens
creation of an optical image using a single lens
incandescent lamps
light sources which produce thermal radiation from an electrically heated filament
index matching fluids
fluids with a refractive index similar to that of other optical materials, typically used for suppressing light reflections
infrared cameras
cameras which work with infrared light
infrared detectors
detectors for infrared light
infrared emitters
devices emitting infrared radiation
infrared light
invisible light with wavelengths roughly between 750  nm and 1  mm
infrared optics
optical elements and systems which are suitable for infrared light
infrared viewers
instruments for observing scenes with infrared light
inhomogeneous broadening
the increase in the linewidth of an atomic transition caused by effects which act differently on different radiating or absorbing atoms
inhomogeneous saturation
gain saturation effects which alter the shape of the gain spectrum
injection locking
a technique for enforcing operation of a laser on a certain optical frequency by injecting light with that frequency into the laser resonator
injection seeding
a technique for enforcing narrowband operation of a laser or optical parametric oscillator by injecting light into its resonator
insertion loss
power losses due to insertion of a device
instantaneous frequency
temporal derivative of the oscillation phase divided by 2π
integrated optics
the technology dealing with the construction of photonic integrated circuits
integrating spheres
devices with a diffusely reflecting inner surface, used for certain optical measurements
intensity modulators
devices which can be used to modulate the intensity (power) of a light beam
intensity noise
noise of the optical intensity or power of a laser beam
interference
a range of phenomena associated with the superposition of waves
interferometers
optical devices utilizing the phenomenon of interference
interlocks
safety devices for automatically switching off a laser power or interrupting a laser beam
intermodal dispersion
the phenomenon that the group velocity of light propagating in a waveguide structure depends on the waveguide mode
intracavity frequency doubling
frequency doubling with a nonlinear crystal within the laser resonator
intracavity laser absorption spectroscopy
a method of laser spectroscopy, based on the evolution of the optical spectrum of intracavity radiation under the influence of narrowband absorption features
irradiance
the radiant flux (optical power) per unit area

J

K

Kelly sidebands
sidebands in the spectrum of certain mode-locked lasers, related to a periodic disturbance of soliton pulses in the laser resonator
Kerr effect
a nonlinear interaction of light in a medium with an instantaneous response, related to the nonlinear electronic polarization
Kerr lens
a lensing effect arising from the Kerr nonlinearity
Kerr lens mode locking
a technique for mode locking a laser, exploiting nonlinear self-focusing
Kramers–Kronig relations
mathematical relations between absorption coefficient and refractive index of transparent media
Kuizenga–Siegman theory
a theory predicting the durations of pulses from actively mode-locked lasers

L

lamp-pumped lasers
lasers which are pumped with flash lamps or arc lamps, or in rare cases with tungsten halogen lamps
large-core fibers
optical fibers with relatively large fiber core
large mode area fibers
optical fibers with relatively large mode areas and a single transverse mode or only a few modes
laser absorption spectroscopy
methods of absorption spectroscopy using laser light
laser applications
applications involving laser devices
laser beam characterization
methods of assessing detailed properties of laser beams
laser beams
light beams propagating dominantly in one direction
laser cooling
a variety of techniques for reducing the temperature (i.e., the random motion) of small particles such as atoms or ions
laser cooling units
devices used for cooling of high-power lasers
laser crystals
transparent crystals with laser-active dopants, used as laser gain media
laser design
the design of a laser device, or the process of working out such a design
laser development
the process of developing a laser device
laser diode collimators
devices for collimating the output of laser diodes
laser diode drivers
electronic current supplies for one or several laser diodes
laser diode modules
modules containing diode lasers, and possibly also some optics, cooling devices, electrical elements, etc.
laser diodes
semiconductor lasers with a current-carrying p–n junction as the gain medium
laser dynamics
the temporal evolution of quantities such as the optical power and gain in a laser
laser guide stars
small bright spots in the sky, generated with laser beams for use in astronomy with adaptive optics imaging
laser heads
assemblies containing a mounted gain medium and means for pumping and cooling, or the complete optical parts of a laser, or assemblies for directing a laser beam to a workpiece
laser-induced breakdown
a kind of spark, initiated by intense laser light
laser-induced damage
damage of optical components induced by laser radiation
laser light
light generated with a laser device
laser line optics
optical elements made specifically for certain laser lines
laser lines
narrow spectral lines obtained from lasers
laser marking
a group of methods for labeling materials with lasers
laser microscopy
a technique for generating microscopic images by scanning objects with a laser
laser mirrors
high-quality mirrors used in laser resonators and other optical setups
laser modeling
the investigation of phenomena and quantitative relations in lasers, using theoretical models and computational methods
laser noise
fluctuations of various parameters of laser light, such as the optical power and phase
laser optics
optical elements and systems used with lasers - either in lasers or for transmitting and manipulating laser light
laser physics
the field of physics dealing with phenomena in lasers
laser pointers
devices used for pointing at items with laser beams
laser rangefinders
devices for measuring distances to objects, using lasers
laser resonators
optical resonators serving as basic building blocks of lasers
laser safety
safety of the use of laser devices
laser specifications
descriptions of essential properties of a laser system, such as performance parameters, dimensions, and operating conditions
laser speckle
light patterns resulting from the reflection of coherent light at rough surfaces
laser spectroscopy
methods where the interaction of laser light with matter is utilized
laser threshold
an operation condition of a laser where laser emission just starts to occur
laser transitions
optical transitions where stimulated emission is used to obtain optical amplification
laser viewing cards
cards used for visualizing laser beams in non-visible spectral regions
lasers
devices generating visible or invisible light, based on stimulated emission of light
lasing without inversion
the quantum-optical phenomenon of laser emission occurring without the presence of a population inversion
leaky modes
a concept for quasi-bound waveguide modes with propagation losses due to leakage into the cladding
lenses
transparent optical devices affecting the wavefront curvature of light
LIDAR
a technique for acquiring 3D images with laser pulses
light-emitting diodes
semiconductor diodes that emit light via electroluminescence
light forces
forces associated with absorption, reflection or scattering of light
linewidth
width of the spectrum of a light beam or an absorption feature
linewidth enhancement factor
a parameter quantifying the amplitude–phase coupling in a laser
liquid crystal displays
displays based on liquid crystal modulators
liquid crystal modulators
optical modulators which are based on liquid crystals
loupes
optical lenses held close to the eye which are used for viewing objects with some moderate magnification
lower-state lifetime
the lifetime of the population of the lower laser level in the absence of radiation
LP modes
linearly polarized modes of optical fibers with radially symmetric index profiles in the approximation of weak guidance
luminescence
light emission which is not caused by heating
luminous efficacy and efficiency
the luminous flux of a light source divided by its the radiant power or its electrical consumption (efficacy); the ratio of reached efficacy to the theoretically possible efficacy (luminous efficiency)
luminous flux
a measure of the perceived power of light, e.g. from a light source
Lyot filters
optical filter devices based on birefringence, exhibiting a wavelength-dependent transmission

M

M2 factor
a parameter for quantifying the beam quality of laser beams
magnification
the enlargements of images
magnifying glasses
optical lenses held above objects for viewing them with some moderate magnification
master laser
a single-frequency laser used for injection locking one or several other lasers
master oscillator fiber amplifier
a laser system containing a fiber amplifier for boosting the output power
master oscillator power amplifier
a laser system consisting of a seed laser and a laser amplifier for boosting the output power
McCumber theory
a theory applied to absorption and emission properties of laser gain media, in particular to solid-state media
mechanical fiber splices
permanent or semi-permanent connections of optical fibers which are held in place with mechanical means
medical lasers
laser systems used for medical applications
metal–semiconductor–metal photodetectors
fast photodetector devices based on metal–semiconductor (Schottky) contacts
metal-coated mirrors
mirrors where the reflection is obtained from a metal coating
metastable states
excited states (particularly electronic states in laser gain media) which have a relatively long lifetime due to slow radiative and non-radiative decay
Michelson interferometers
an interferometer with two separate arms and a single beam splitter
microbends of fibers
microscopic bends of optical fibers
microchannel plates
electron amplifiers with many spatial channels
microchip lasers
solid-state lasers with a quasi-monolithic setup
microlens arrays
one- or two-dimensional arrays of microlenses, used e.g. in Shack-Hartmann wavefront sensors
microlenses
optical lenses of particularly small diameter, e.g. below 1 mm
microscope objectives
the optical component of a microscope which is next to the observed object
microscopes
instruments for seeing very small objects
mid-infrared fibers
optical fibers for use with mid-infrared light
mid-infrared laser sources
light sources emitting mid-infrared radiation in the form of a laser-like beam
mirror substrates
substrates on which coatings for laser mirrors can be deposited
mirrors
devices which reflect light
modal bandwidth
the maximum optical bandwidth (limited by intermodal dispersion) which can be used in a telecom fiber
mode cleaner cavities
optical cavities used as filters for improving the beam quality of laser beams
mode cleaners
devices which can improve the beam quality of laser beams
mode competition
the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects
mode coupling
a concept for describing and calculating light propagation in certain situations, e.g. involving nonlinear interactions
mode division multiplexing
a multiplexing technique for data transmission in fibers, where different fiber modes are used for different channels
mode field converters
optical devices which allow for efficient coupling between modes of different sizes
mode hopping
the phenomenon that a laser exhibits sudden jumps of optical frequency, which are associated with transitions between different modes of its resonator
mode-locked diode lasers
diode lasers which are actively, passively or hybrid mode-locked for generating ultrashort pulses
mode-locked fiber lasers
ultrafast fiber lasers which are actively or passively mode-locked for generating ultrashort pulses
mode-locked lasers
lasers which emit ultrashort pulses on the basis of the technique of mode locking
mode locking
a group of techniques for generating ultrashort pulses in lasers
mode locking devices
devices for achieving mode locking of a laser
mode matching
the precise spatial matching of the electric field distributions of laser beams and resonator modes or waveguide modes
mode radius
a measure of the transverse extent of a laser mode or laser beam
modes
self-consistent electric field distributions in waveguides, optical resonators or in free space
modes of laser operation
a mode of operation such as continuous-wave operation, Q-switched or mode-locked operation
modulation depth
a relative modulation amplitude, or (for a saturable absorber) the maximum change in absorption
molecular lasers
gas lasers where the laser-active gas consists of molecules rather than separate atoms or ions
monochromatic light
light with a single optical frequency
monochromators
optical devices which transmit only light in an adjustable small range of optical wavelengths
monolithic solid-state lasers
solid-state lasers where the whole laser resonator consists only of one piece of crystal or glass
multi-core fibers
optical fibers containing more than one fiber core
multi-phonon transitions
transitions between electronic levels of atoms or ions in solid media, involving the emission of multiple phonons
multimode fibers
fibers supporting more than one guided mode per polarization direction
multipass amplifiers
an amplifier with multiple passes through the gain medium
multipass gas cells
a cell which allows to realize multiple passes of a laser beam through a gas
multiphonon absorption
absorption processes involving multiple phonons
multiphoton absorption
absorption processes where multiple photons are absorbed simultaneously
multispectral imaging
imaging which provides images with multiple spectral channels

N

nanofibers
optical fibers with transverse dimensions below one micrometer
nanosecond lasers
lasers emitting optical pulses with nanosecond durations
narrow-linewidth lasers
single-frequency lasers with a narrow optical emission spectrum
near field and far field
the optical field either close to a source or object or far from it
neodymium-doped gain media
laser gain media containing laser-active neodymium ions
neutral density filters
optical attenuators with an approximately constant attenuation in a substantial wavelength range
night vision devices
instruments which allow viewing or imaging under conditions of very low light levels
nodal points
points of an optical system for which an incoming ray, directed at a nodal point, leaves the system with the same direction.
noise eaters
devices for reducing the intensity noise of an optical beam by automatically adjusting the degree of power attenuation
noise-equivalent power
the input power to a detector which produces the same signal output power as the internal noise of the device
noise figure
a measure of the excess noise added in an amplifier
noise specifications
specifications for the noise properties of lasers, for example
non-radiative transitions
transitions between energy levels of atoms or ions which are not associated with the emission of light
nonclassical light
light with properties which can be explained only within quantum optics
noncritical phase matching
a technique of phase matching which does not require a critical angular adjustment
nonlinear crystal materials
crystal materials exhibiting an optical nonlinearity, usually of χ(2) type
nonlinear frequency conversion
the conversion of input light to light of other frequencies, using optical nonlinearities
nonlinear index
a parameter for quantifying the Kerr nonlinearity of a medium
nonlinear optics
the part of optics dealing with optical nonlinearities and their applications
nonlinear polarization
the part of the light-induced electric polarization which depends nonlinearly on the electric field of the light
nonlinear polarization rotation
the phenomenon that the change in the polarization direction of light in a fiber can depend on the optical intensity
nonlinear pulse distortion
distortion of the spatial, temporal or spectral characteristics of an optical pulse, resulting from optical nonlinearities
nonlinearities
optical phenomena involving a nonlinear response to a driving light field
nonplanar ring oscillators
a type of monolithic single-frequency lasers, based on a nonplanar ring resonator
numerical aperture
sine of the maximum angle of an incident beam of some optical device, or the sine of the acceptance angle of a waveguide or fiber

O

object and image space
mathematical spaces for the description of optical elements and systems
objectives
optical elements for collecting light from inspected objects, or for sending light to objects
ocular lenses
lenses or lens systems as used in telescopes and microscopes, for example, close to the observing eye
optical aberrations
degradations of optical images caused in imaging instruments
optical amplifiers
devices for amplifying the power of light beams
optical apertures
structures which limit the propagation of light
optical attenuators
devices which can reduce the optical power e.g. of a light beam
optical breadboards
portable platforms for optical prototyping and for mounting modules on optical tables
optical choppers
devices for periodically interrupting light beams
optical clocks
time measurement devices based on optical frequency standards
optical clockworks
devices which can phase-coherently relate optical frequencies to microwave frequencies
optical coherence tomography
an optical imaging technique where longitudinal resolution is achieved based on interference effects with low-coherence light
optical data transmission
the transmission of information using light beams, e.g. in fibers
optical delay lines
an optical setup which is used to introduce a time delay for a light beam
optical density
a logarithmic measure of the power attenuation, or the refractive index
optical domes
strongly curved optical windows
optical energy meters
instruments for measuring optical pulse energies
optical fabrication
the fabrication of optical components like lenses, prisms and mirrors
optical fiber communications
the technology of transmitting information through optical fibers
optical filters
devices with a wavelength-dependent transmission or reflectance
optical flats
plates with one or two surfaces of very high optical quality and flatness
optical frequency
the oscillation frequency of the electric field of light
optical frequency standards
frequency standards using optical transitions in atoms, ions or molecules
optical glasses
glasses of high quality, made for optical applications
optical heterodyne detection
a sensitive method of photodetection, reaching the standard quantum limit even for weak signal powers
optical intensity
optical power per unit area
optical materials
materials used for optical components
optical metrology
the science and technology of performing measurements with light
optical modulators
devices allowing one to manipulate properties of light beams, such as the optical power or phase
optical molasses
an arrangement of laser beams used for cooling atoms or ions
optical parametric amplifiers
optical amplifiers based on parametric nonlinear interactions
optical parametric chirped-pulse amplification
parametric amplification of chirped ultrashort pulses
optical parametric generators
light sources based on parametric amplifiers with no signal or idler input
optical parametric oscillators
coherent light sources based on parametric amplification within an optical resonator
optical phase
the phase of the electric field of an optical wave, usually considered in the time domain
optical power
the energy per unit time, e.g. transported by a laser beam, or a focusing power
optical power meters
instruments for optical power measurements, based e.g. on heating of an absorber structure or a photodiode
optical power monitors
devices for monitoring optical powers in free-space light beams or in optical fibers
optical profilometers
optical non-contact instruments for precisely measuring surface profiles
optical pumping
electronically exciting a medium with light, or specifically populating certain electronic levels
optical rectification
a nonlinear process where an optical field generates a quasi-DC nonlinear polarization
optical refrigeration
a technique for cooling macroscopic samples via laser-induced fluorescence
optical resonators
arrangements of optical components which allow a beam of light to circulate
optical sampling
a technique of sampling signals using laser light, often in the form of ultrashort pulses
optical slits
narrow openings through which light can be transmitted
optical spectrum
the decomposition of the power or energy of light according to different wavelengths or optical frequencies
optical spectrum analyzers
instruments for measuring optical spectra
optical strain sensors
sensors for mechanical strain based on optical technology, e.g. with fiber Bragg gratings
optical tables
platforms for building optical devices with suppression of vibrations
optical temperature sensors
temperature sensors based on optical technology, e.g. with fiber Bragg gratings
optical thickness
a measure either for the absorption or the effective optical path length of a sample
optical time-domain reflectometers
instruments for measuring the spatially resolved reflectivities and losses in optical fibers
optical tweezers
arrangements for capturing and moving particles with laser beams
optical windows
flat transparent plates with optical quality, used for protection against the environment
optics
the science and technology dealing with the properties and the propagation of light
opto-isolators
optoelectronic devices for signal transmission with electrical isolation between two electrical circuits
opto-mechanics
mechanical elements used in optics
optoelectronics
the technology of electronic devices that interact with light
orientation-patterned semiconductors
semiconductors containing a periodic pattern of domain orientation
oscillators
a device producing an oscillation – in contrast to an amplifier, for example
output couplers
partially transparent laser mirrors, used for extracting output beams from laser resonators
output coupling efficiency
a factor influencing the power efficiency of a laser, taking into account intracavity losses

P

p–i–n photodiodes
photodiodes with a p–i–n semiconductor structure
parabolic mirrors
mirrors with the surface shape of a paraboloid, or at least a parabolic cross section
parabolic pulses
pulses with a parabolic intensity profile
parametric amplification
a process of optical amplification based on a parametric nonlinearity
parametric fluorescence
emission of light by a parametric amplifier without signal or input inputs
parametric nonlinearities
instantaneous optical nonlinearities based on the χ(2) or χ(3) nonlinear tensor of a transparent medium
parasitic lasing
unwanted laser operation in a laser or amplifier device
paraxial approximation
a frequently used approximation, essentially assuming small angular deviations of the propagation directions from some beam axis
passive mode locking
a technique of mode locking, based on a saturable absorber inside the laser resonator
peak power
maximum optical power of a pulse
periodic poling
a technique for achieving quasi-phase matching of nonlinear interactions in a transparent crystal material
phase matching
a group of techniques for achieving efficient nonlinear interactions in a medium
phase-matching bandwidth
the width of an optical frequency range in which some process (e.g. a nonlinear interaction) can be efficient due to more or less precise phase matching
phase modulators
devices for manipulating the phase of a laser beam
phase noise
noise of the optical phase of a beam or of an electric signal
phase shift method for distance measurements
a method for optical distance measurements
phase velocity
the velocity with which phase fronts propagate in a medium
phonons
quantized microscopic vibrations in solid media
phosphate glasses
certain glasses from which certain optical fibers and laser gain media can be made, for example
phosphorescence
a luminescence which lasts for a relatively long time after excitation of a medium
phosphors
substances which exhibit luminescence, used mostly for illumination and displays
photo cameras
optical instruments for taking photographic images
photocathodes
an electrode where electrons can be released based on the photoelectric effect
photochromic materials
optical materials which exhibit increased light absorption when being exposed to light
photoconductive detectors
photodetectors which exhibit a reduction in electrical resistance caused by incoming light
photoconductive sampling
a technique of optical sampling with photoconductive switches
photoconductive switches
electric switches controlled by light via photo-induced conductivity
photocurrent
an electric current which is induced by incident light in a photodetector
photodarkening
the phenomenon that the optical power losses in a medium can grow when the medium is irradiated with light at certain wavelengths
photodetectors
devices used for the detection of light
photodiode arrays
typically linear arrays of photodiodes, provided as compact devices
photodiodes
semiconductor devices with a p–n or p–i–n structure for the detection of light
photoelectric effect
the emission or excitation of electric carriers related to the absorption of light
photoemissive detectors
photodetectors which are based on the external photoelectric effect
photographic objectives
objective lenses used for photography
photoluminescence
emission of light which is caused by the irradiation of a material with other light
photometers
instruments for measuring light properties such as irradiance or illuminance, or optical properties of objects
photometry
the science and technology of measuring properties of light related to its perceived brightness for human eyes
photomultipliers
photodetection devices based on the photoelectric effect and charge multiplication by secondary emission of electrons
photon counting
photodetection at low light levels where single photon absorption events are counted
photonic bandgap fibers
optical fibers where light is guided based on a photonic bandgap effect
photonic crystal fibers
specialty optical fibers with a built-in microstructure, in most cases consisting of small air holes in glass
photonic integrated circuits
integrated circuits with optical functions
photonic metamaterials
nanostructured materials with special optical properties
photonics
the science and technology of light
photons
quanta of light energy
phototransistors
a type of semiconductor-based photodetector with internal current amplification for higher responsivity
phototubes
gas-filled or vacuum tubes which can be used as photodetectors
picosecond lasers
lasers emitting pulses with picosecond durations
pinholes
circular holes, typically with diameters well below one millimeter
planar waveguides
waveguide structures guiding light only in one dimension
plane waves
waves with plane wavefronts
plastic optical fibers
optical fibers made of polymer materials
plastic optics
optical elements made of organic polymer materials
Pockels cell drivers
electronic devices for driving Pockels cells
Pockels cells
electro-optic devices, used for building modulators
Pockels effect
the phenomenon that the refractive index of a medium exhibits a modification which is proportional to the strength of an applied electric field (linear electro-optic effect)
polarization beam combining
a technique for combining two polarized laser beams
polarization beat length
the period of interference effects in a birefringent medium
polarization-maintaining fibers
specialty optical fibers with strong built-in birefringence, preserving the properly oriented linear polarization of an input beam
polarization mode dispersion
the polarization dependence of the propagation characteristics of light waves in optical fibers
polarization of light
direction of the electric field oscillation of a light beam
polarization waves
patterns of electric (or magnetic) polarization of a medium which are usually caused by light waves
polarizers
optical filters where the light transmission depends strongly on the direction of polarization
polychromatic light
light with multiple optical frequencies
population inversion
a state of a medium where a higher-lying electronic level has a higher population than a lower-lying level
position-sensitive detectors
photodetectors which can detect the position of a light spot in one or two dimensions
power density
intensity or power per unit frequency interval
power over fiber
delivery of power for electronic devices via light in an optical fiber
power scaling of lasers
a procedure for substantially increasing the output power of lasers
power spectral density
optical power or noise power per unit frequency interval
principal dispersion
a crude measure for chromatic dispersion of a transparent optical material
principal points and principal planes
planes from which rays appear to be deflected in an optical system according to paraxial optics, and the intersections of those planes with the optical axis
prism pairs
combinations of two prisms, mostly used for dispersion compensation
prisms
transparent optical devices which refract or reflect light
propagation constant
a mode- and frequency-dependent quantity describing the propagation of light in a medium or waveguide
propagation losses
losses of optical energy during propagation of light
pulse characterization
the measurement of various properties of an optical pulse
pulse compression
linear or nonlinear techniques for reducing the durations of optical pulses
pulse duration
the duration of an optical pulse
pulse energy
energy of an optical pulse
pulse front tilt
the phenomenon that the arrival time of an ultrashort pulse varies across the beam profile
pulse generation
the generation of light pulses (flashes)
pulse pickers
electrically controlled optical switches used for extracting single pulses from a pulse train
pulse propagation modeling
working with physical models describing the propagation of ultrashort pulses e.g. in lasers or optical fibers
pulse repetition rate
the number of pulses emitted per second e.g. by a mode-locked or Q-switched laser
pulse shapers
devices which can be used to modify the temporal or spectral shape of ultrashort light pulses
pulse stretchers
devices for temporally stretching ultrashort light pulses
pulsed laser deposition
a method for fabricating thin films on substrates by evaporating materials with laser pulses
pulsed lasers
lasers emitting light in the form of pulses
pulses
flashes of light
pump–probe measurements
techniques for investigating ultrafast phenomena, where a pump pulse excites a sample and a probe pulse is used for probing the sample after an adjustable delay time
pump absorption
the absorption of pump light, e.g. in a laser or a laser amplifier
pump chambers
a part of a lamp-pumped solid-state laser, containing a laser rod, one or more lamps, reflectors, a cooling arrangement and possibly other parts
pump depletion
reduction in the transmitted pump power in a laser or nonlinear conversion device
pump parameter
the ratio of pump power and threshold pump power of a laser
pyroelectric detectors
light detectors based on the pyroelectric effect

Q

Q factor
a measure of the damping of resonator modes
Q-switched lasers
lasers which emit optical pulses, relying on the method of Q switching
Q-switched mode locking
an operation regime of mode-locked lasers with strong fluctuations of the pulse energy
Q switches
optical switches which are typically used for generating nanosecond pulses in lasers
Q switching
a method for obtaining energetic pulses from lasers by modulating the intracavity losses
Q-switching instabilities
instabilities of passively mode-locked lasers where the pulse energy is unstable
quantum cascade lasers
semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region
quantum cryptography
methods to encrypt information securely, relying on quantum-mechanical phenomena
quantum defect
energy difference between pump and laser photons
quantum dots
microscopic structures confining charge carriers in three dimensions
quantum efficiency
percentage of input photons which contribute to a desired effect
quantum electronics
an area of physics which is the basis of optoelectronics
quantum key distribution
methods for the secure distribution of encryption keys
quantum noise
noise arising from quantum fluctuations
quantum optics
the part of optics which deals with quantum effects
quantum photonics
the science and technology using quantum optics for applications like quantum communications, quantum computing and quantum metrology
quantum wells
thin layers confining carriers in one dimension
quarter-wave mirrors
dielectric mirrors based on a sequence of quarter-wave layers
quasi-continuous-wave operation
an operation mode of lasers (e.g. diode bars) where the pump power is switched on for short time intervals in order to limit thermal effects
quasi-phase matching
a technique of achieving a kind of phase matching, using a periodic structure
quasi-soliton pulses
soliton-like pulses in lasers or fiber-optic links
quenching
the reduction or limitation of an excited-state population, mostly by unwanted effects

R

Rabi oscillations
oscillations of level populations (or quantum mechanical probability amplitudes) under the influence of an incident light field
radiance
the optical power per unit area and solid angle
radiant energy
energy of some electromagnetic radiation which is emitted, transmitted or absorbed during some interval of time
radiant flux
radiant energy per unit time which is emitted, transmitted, reflected or received by an object
radiation-balanced lasers
lasers where the heat generation in the gain medium is compensated by optical refrigeration in the same medium
radiation pressure
a mechanical pressure resulting from incident light or other radiation
radiative lifetime
lifetime of an electronic state in the (hypothetical) situation where only radiative processes depopulate that level
radiometry
the science and technology of measuring properties of electromagnetic radiation, including light
Raman amplifiers
optical amplifiers based on Raman gain
Raman gain
optical gain arising from stimulated Raman scattering
Raman lasers
lasers based on Raman gain rather than on laser gain from stimulated emission
Raman scattering
a nonlinear scattering process involving optical phonons
Raman spectroscopy
optical spectroscopy based on spontaneous or stimulated Raman scattering
rare-earth-doped fibers
optical glass fibers which are doped with rare earth ions
rare-earth-doped gain media
laser gain media which are doped with rare earth ions
rate equation modeling
a method of modeling the dynamics of laser gain media
Rayleigh length
the distance from a beam waist where the mode radius increased by a factor square root of 2
Rayleigh scattering
scattering of light at scattering centers which are much smaller than the wavelength
reciprocity method
a spectroscopic method which is often used for obtaining the scaling of emission spectra of laser gain media
recirculating fiber loops
fiber loops in which light can circulate many times
recoil limit
a lower limit for the temperature which can be reached with some laser cooling techniques
red lasers
lasers emitting red light
reference cavities
optical cavities serving as a kind of frequency reference
reflectance
the ratio of reflected optical power to the incident optical power at some reflecting object
reflectivity
the ratio of reflected optical power to the incident optical power at a surface
refraction
the change of the propagation direction when a wave comes from one medium into another one
refractive index
a measure of the reduction in the velocity of light in a medium
regenerative amplifiers
optical amplifiers with a resonator in which a light pulse can do multiple round trips before being coupled out
relative intensity noise
noise of the optical intensity (or actually power), normalized to its average value
relaxation oscillations
small mutually coupled oscillations of the laser power and laser gain around their steady-state values
resonant frequency doubling
frequency doubling with a nonlinear crystal placed in a resonant enhancement cavity
resonator design
the description of the optical components from which an optical resonator is made, and their exact arrangement
resonator modes
modes of an optical or microwave resonator
responsivity
photocurrent per unit optical power incident on a photodetector
retardance
the difference in optical phase shifts between two polarization directions, e.g. for light transmitted through a sample or reflected at a surface
retroreflectors
optical devices which reflect light in a direction which is parallel to the incident beam
return loss
a measure for how much reflected light is attenuated
RGB sources
source of red, green and blue light, which is usually provided in the form laser beams
ring lasers
lasers with a ring resonator
ring resonators
optical resonators where light can independently circulate in two different directions
rod lasers
solid-state lasers with a rod-shaped gain medium
rod lenses
cylindrical lenses with the geometrical form of a cylinder
ruby lasers
solid-state lasers based on a ruby crystal
rugate filters
dielectric interference filters with a continuous variation of the refractive index

S

saturable absorbers
light absorbers with a degree of absorption which is reduced at high optical intensities
saturable Bragg reflectors
a special type of saturable semiconductor absorbers
saturation energy
a measure of the incident optical pulse energy required for achieving significant saturation of an absorber or a gain medium
saturation power
a measure of the incident optical power required for achieving significant saturation of an absorber or a gain medium
scanning lenses
lens systems which are designed for use in laser scanners
scattering
processes where light is sent in other directions, usually but not always in random directions
Schawlow–Townes linewidth
linewidth of a single-frequency laser with quantum noise only
sech2-shaped pulses
pulses with a temporal intensity profile which has the shape of a sech2 function
seed lasers
lasers which are used for generating some seed light into an amplifier or another laser
self-focusing
focusing of a beam in a transparent medium, caused by the beam itself through a nonlinear process in the medium
self-heterodyne linewidth measurement
a technique for laser linewidth measurements, based on a beat note between the beam and a delayed version of itself
self-phase modulation
nonlinear phase modulation of a beam, caused by its own intensity via the Kerr effect
self-starting mode locking
mode locking which is quickly achieved after turning on a laser, without external intervention
self-terminating laser transitions
laser transitions which can not be continuously operated due to the accumulation of population in the lower laser level
Sellmeier formula
an equation for calculating the wavelength-dependent refractive index of a medium
semiconductor lasers
lasers based on semiconductor gain media
semiconductor optical amplifiers
optical amplifiers based on semiconductor gain media
semiconductor saturable absorber mirrors
saturable semiconductor absorber devices acting as nonlinear mirrors
sensitivity
(different meanings, depending on the context)
Shack–Hartmann wavefront sensors
wavefront sensors which are based on a microlens array
shot noise
quantum-limited intensity noise
side pumping
a technique of pumping a solid-state laser in directions which are approximately transverse to its beam direction
signal-to-noise ratio
the ratio of signal power to noise power in a detector
silica fibers
optical fibers based on fused silica or related materials
silicon photonics
photonic technology based on silicon chips
single-atom lasers
lasers with only a single atom as the gain medium
single-crystal fibers
long and thin rods made of monocrystalline materials
single-frequency lasers
lasers emitting radiation in a single resonator mode
single-frequency operation
an operation mode of a laser where only a single resonator mode acquires a significant power
single-mode fibers
optical fibers supporting only a single guided mode per polarization direction
single-mode operation
operation of a laser on axial (longitudinal) resonator modes only, or even on a single axial mode
single-polarization fibers
optical fibers which transmit only light with a certain polarization
Sisyphus cooling
a mechanism for laser cooling of atoms or ions
slab lasers
solid-state bulk lasers where the gain medium has the form of a slab
slave laser
a laser which is injection-locked to another laser
slope efficiency
differential power efficiency of a laser
small-signal gain
optical gain for small signal intensities, where no amplifier saturation occurs
solar-blind photodetectors
photodetectors which are insensitive to visible light light but react to ultraviolet light
solid-state lasers
lasers based on solid-state gain media (usually ion-doped crystals or glasses)
soliton mode locking
a mechanism for laser mode locking based on soliton pulses
soliton period
the distance over which higher-order solitons reproduce their temporal and spectral shape
solitons
pulses with a certain balance of nonlinear and dispersive effects
space division multiplexing
a multiplexing technique for optical data transmission where multiple spatial channels are utilized
spatial hole burning
a distortion of the gain shape in a laser medium (or the loss spectrum in a saturable absorber medium), caused by saturation effects of a standing wave
spatial walk-off
the phenomenon that the intensity distribution of a beam in an anisotropic crystal drifts away from the direction of the wave vector
specialty fibers
optical fibers with special designs or materials
spectral beam combining
a class of methods for beam combining, based on wavelength-sensitive beam combiners
spectral brightness
brightness per unit optical bandwidth
spectral imaging
imaging which also provides spectral information
spectral lines
narrow spectral features related to emission or absorption lines of certain substances
spectral phase
the phase of the electric field in the frequency domain
spectral phase interferometry
a class of interferometric methods for pulse characterization, where interference in the frequency domain is exploited
spectral response of a photodetector
the range of optical wavelengths for which a photodetector is sensitive
spectrograms
a type of diagrams which visualize optical or other signals
spectrographs
instruments for spectrally analyzing light
spectrometers
devices for separating spectral components
spectrophotometers
instruments for measuring the wavelength-dependent transmittance or reflectance of solutions or solid objects
spectroscopy
methods where the interaction of light with matter is utilized, e.g. for detecting substances or measuring concentrations
spherical aberrations
optical aberrations resulting from spherical optical surfaces
spiking
the tendency of some lasers to emit spikes of output power, particularly after being switched on
spontaneous emission
a quantum effect, causing the spontaneous decay of excited states of atoms or ions
squeezed states of light
nonclassical states of light with noise below the standard quantum limit in one quadrature component
stability zones of optical resonators
parameter regions of an optical resonator where the resonator is geometrically stable
stabilization of lasers
measures applied to lasers in order to improve their stability in terms of output power, optical frequency, or other quantities
standard quantum limit
a limit for noise levels set by quantum mechanics
standard spectral lines
spectral lines which are often used as wavelength references
step-index fibers
optical fibers with a step-index refractive index profile
stimulated emission
a quantum effect, where photon emission is triggered by other photons
Stokes shift
the reduction in photon energy in certain processes
streak cameras
devices for measuring the power of an optical pulse versus time
sum and difference frequency generation
nonlinear processes generating beams with the sum or difference of the frequencies of the input beams
supercontinuum generation
a nonlinear process for strong spectral broadening of light
superfluorescence
collective emission of radiation by an ensemble of excited atoms or ions
superluminal transmission
transport of some quantity with a velocity exceeding that of light in vacuum
superluminescence
fluorescence which is enhanced by laser amplification
superluminescent diodes
broadband semiconductor light sources based on superluminescence
superluminescent sources
optical sources based on superluminescence
supermirrors
laser mirrors with a very high reflectivity
superradiance
collective emission of an ensemble of atoms or ions after coherent excitation
surface-emitting semiconductor lasers
semiconductor lasers where the generated light propagates in the direction perpendicular to the wafer surface
synchronization of lasers
the synchronization of light pulses generated with lasers
synchronous pumping
pumping of a laser or an optical parametric oscillator with short light pulses in synchronism with the emitted pulses

T

tapered amplifiers
optical amplifiers which contain a tapered section, where the beam area is gradually increased
tapered fibers
optical fibers which are over some length stretched out to a very small diameter
tapered laser diodes
laser diodes containing a tapered region for achieving a higher beam quality
telecentric lenses
lenses or objectives having the entrance or exit pupil at infinity
telecom fibers
optical fibers for use in optical fiber communications
telescopes
optical instruments for viewing distant objects, or sometimes more generally afocal systems
temporal walk-off
the loss of temporal overlap of pulses, caused by group velocity mismatch
terahertz detectors
detectors for terahertz radiation
terahertz radiation
electromagnetic radiation with frequencies around 0.1 THz to 10 THz
terahertz sources
sources of electromagnetic radiation with frequencies around 0.1 THz to 10 THz
thermal detectors
light detectors which are based on measuring light-induced temperature rises
thermal imaging
imaging based on thermal radiation
thermal lensing
a lensing effect induced by temperature gradients
thermal radiation
electromagnetic radiation which results from thermal emission
thin-disk lasers
solid-state bulk lasers having a very thin disk of laser-active material as the gain medium
thin-film polarizers
optical polarizers based on a multilayer dielectric coating
third-order dispersion
chromatic dispersion related to a third-order dependence of the phase change on the frequency offset
threshold pump power
the pump power at which the laser threshold is reached
thresholdless lasers
lasers with a threshold power which is virtually zero
time–bandwidth product
product of temporal and spectral width of a pulse
time division multiplexing
a multiplexing technique operating in the time domain
time-of-flight measurements
distance measurements based on measuring the time of flight of a light pulse
time-resolved spectroscopy
spectroscopy where time-dependent signals are measured
timing jitter
fluctuations of the temporal positions of pulses
timing phase
a phase reflecting timing deviations by relating them to the pulse period
titanium–sapphire lasers
lasers based on a Ti:sapphire gain medium
total internal reflection
the phenomenon that light incident on the boundary between two transparent media is fully reflected if the light is coming from the optical more dense medium and the angle of incidence is sufficiently large
transform limit
a limit for the time–bandwidth–product of an optical pulse
transition cross sections
material parameters for quantifying the likelihood or rate of optical transition events
transition-metal-doped gain media
laser gain media which are doped with transition metal ions
transmissivity
the ratio of transmitted optical power to the incident optical power at a surface
transmittance
the ratio of transmitted optical power to the incident optical power at some partial transparent object
triangulation
a method for distance measurements
tunable lasers
lasers the output wavelengths of which can be tuned
tunable optical filters
optical filters with tunable filter parameters, e.g. concerning center wavelength of pass band
tungstate lasers
solid-state lasers based on tungstate crystals as gain media
twisted-mode technique
a technique for achieving single-frequency operation of a laser, based on quarter-wave plates on both sides of the gain medium
two-photon absorption
a nonlinear absorption process where two photons are simultaneously absorbed
Twyman–Green interferometers
an interferometer similar to a Michelson interferometer, but with expanded beams

U

ultrafast amplifiers
amplifiers for ultrashort optical pulses
ultrafast laser physics
the area of laser physics dealing with ultrashort processes
ultrafast lasers
lasers emitting ultrashort pulses
ultrafast optics
the part of optics dealing with ultrafast phenomena
ultrashort pulses
optical pulses with durations of picoseconds or less
ultraviolet lasers
lasers (or other laser-based light sources) generating ultraviolet light
ultraviolet light
invisible light with wavelengths shorter than ≈ 400  nm
ultraviolet optics
optical elements for use with ultraviolet light
unstable resonators
optical resonators which are dynamically unstable with respect to transverse beam offsets
upconversion
a process where light can be emitted with photon energies higher than those of the light generating the excitation
upconversion lasers
lasers relying on upconversion processes
upper-state lifetime
the lifetime of the population of the upper laser level

V

V number
a normalized frequency parameter, which determines the number of modes of a step-index fiber
vacuum noise
quantum noise of the vacuum state, or quantum noise introduced into an optical system via a vacuum input
vanadate lasers
lasers based on rare-earth-doped yttrium, gadolinium or lutetium vanadate crystals, usually Nd:YVO4
variable optical attenuators
optical attenuators with a variable degree of attenuation
VCSEL arrays
arrays containing many VCSEL laser emitters
velocity-matched photodetectors
a type of photodetector where propagation velocities of light and electrical signals are matched in order to achieve a very high detection bandwidth
velocity of light
the velocity with which light propagates in a medium
vertical cavity surface-emitting lasers
a monolithic kind of semiconductor lasers with beam emission perpendicular to the wafer surface
vertical external-cavity surface-emitting lasers
surface-emitting semiconductor lasers with an external laser resonator
vibronic lasers
lasers based on gain media with a large gain bandwidth, caused by a strong interaction of electronic transitions with phonons
vignetting
a reduction of image brightness in peripheral regions
visible lasers
lasers (or other laser-based light sources) emitting visible light
volume Bragg gratings
Bragg gratings written into some bulk material

W

wall-plug efficiency
total electrical-to-optical power efficiency of a laser system
wave optics
the description of optical phenomena based on wave models
wave vector
a vector indicating the direction of wave propagation and the phase delay per unit length
wavefronts
surfaces connecting points with the same phase of a wave
waveguide dispersion
chromatic dispersion arising from waveguiding effects
waveguide lasers
lasers with a waveguide structure in the gain medium
waveguides
spatially inhomogeneous transparent structures for guiding light
wavelength
the spatial period of a plane wave
wavelength division multiplexing
a multiplexing technique working in the wavelength domain
wavelength-swept lasers
lasers which allow for period and fast wavelength tuning
wavelength-tunable light sources
light sources where the optical wavelength can be tuned
wavelength tuning
the manipulation of the output wavelength of an optical device such as a laser or an optical parametric oscillator
wavemeters
devices for precise measurements of laser wavelengths
wavenumber
the phase delay per unit length, or that quantity divided by 2π
waveplates
transparent plates with a defined amount of birefringence, used for modifying the polarization of light
wedge prisms
prisms with a small angle between the end surfaces
white light interferometers
interferometers using broadband light inputs
white light sources
light sources with very broad optical bandwidth
working distance
the distance between an objective and the object

X

X-ray lasers
lasers emitting in the X-ray spectral region

Y

YAG lasers
lasers based on YAG (yttrium aluminum garnet) crystals, usually Nd:YAG or Yb:YAG
yellow and orange lasers
lasers emitting yellow or orange light
YLF lasers
lasers based on YLF (yttrium lithium fluoride) crystals, usually Nd:YLF
ytterbium-doped gain media
laser gain media containing laser-active ytterbium ions

Z

z-scan measurements
a technique for measuring the strength of the Kerr nonlinearity of a material, relying on self-focusing
zero dispersion wavelength
a wavelength where the group delay dispersion of a fiber or a material is zero

List of Formula Symbols

SymbolExplanationSI Units
a core radius of a fiber m
b confocal parameter of a laser beam (twice the Rayleigh length) m
c velocity of light m/s
deff effective nonlinear coefficient of a nonlinear crystal material m/V
dij components of the χ(2) nonlinear tensor m/V
e elementary electric charge C
f focal length (e.g., of a lens) m
f noise frequency Hz
frep pulse repetition rate Hz
fro relaxation oscillation frequency Hz
g gain coefficient 1 or m−1
g(τ) complex degree of coherence 1
gB nonlinear gain coefficient for Brillouin scattering m/W
gR nonlinear gain coefficient for Raman scattering m/W
gss small-signal gain coefficient 1 or m−1
h Planck's constant J s
k wavenumber m−1
kB Boltzmann constant J/K
Δk phase mismatch of a parametric nonlinear process rad/m
lrt round-trip losses of a resonator 1
n refractive index 1
ne extraordinary refractive index 1
neff effective refractive index 1
ng group index 1
no ordinary refractive index 1
n2 nonlinear index m2/W
q complex q parameter (→ Gaussian beams) m
r radius (e.g., in fiber core or distance from beam axis) m
r pump parameter of a laser 1
rij components of the electro-optic tensor m/V
vg group velocity m/s
vp phase velocity m/s
w Gaussian beam radius m
w0 beam radius at the beam waist 1
zR Rayleigh length m
zs soliton period m
A amplitude of pulse envelope
Aeff effective mode area m2
B B integral rad
B noise bandwidth Hz
B brightness W/(m2 rad2)
BPP beam parameter product mm mrad
Dλ dispersion parameter ps/(nm km)
E electric field strength V/m
Ep pulse energy J
Esat saturation energy J
F finesse 1
F noise figure 1
Fsat saturation fluence J/m2
GX(τ) autocorrelation function of some variable X
I optical intensity W/m2
Isat saturation intensity W/m2
Lcoh coherence length m
M2 M2 factor 1
NF Fresnel number 1
Ndop doping concentration m−3
NA numerical aperture 1
P optical power W
P electric polarization of a medium C/m2
Pout output power W
Pp peak power of a pulse W
Psat saturation power of a gain medium or saturable absorber W
Q Q factor 1
R power reflectivity (or reflectance) (of a mirror) 1
R radius of curvature (e.g., of wavefronts) m
R responsivity of a photodiode A/W
ΔR modulation depth of a saturable absorber 1
Sφ(f) power spectral density of a phase value rad2/Hz
T temperature K or °C
Tg group delay s
Toc output coupler transmission 1
Trt round-trip time of an optical resonator s
V V number of a step-index fiber 1
α absorption coefficient m−1
α linewidth enhancement factor 1
α thermal expansion coefficient K−1
β propagation constant rad/m
β2 group velocity dispersion s2/m
γSPM self-phase modulation coefficient rad/(W m)
ε0 electric permittivity of vacuum C/Vm
ε relative electric permittivity 1
η efficiency 1
ηoc output coupling efficiency 1
θ beam divergence half-angle rad
θ polar angle rad
κ thermal conductivity W/(m K)
λ optical wavelength m
λp pump wavelength m
λs signal wavelength m
μ relative magnetic permeability 1
μ0 magnetic permeability of vacuum N/A2
ν optical frequency Hz
νB Brillouin frequency shift Hz
νceo carrier–envelope offset frequency Hz
ν(t) instantaneous frequency Hz
Δν optical bandwidth Hz
ρ walk-off angle rad
σabs absorption cross section m2
σem emission cross section m2
τ2 upper-state lifetime s
τcoh coherence time s
τp pulse duration s
τrad radiative lifetime s
φ azimuth angle rad
φ optical phase or phase shift rad
φG Gouy phase shift rad
φnl nonlinear phase shift from the Kerr effect rad
φt timing phase rad
Δφceo carrier–envelope offset phase rad
χ(2), χ(3) nonlinear tensor 1
ω angular frequency rad/s, s−1
Λ modulation period of a fiber Bragg grating m

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